1. Field of the Invention
The present invention relates to fluororesin fiber paper excellent in heat resistance, chemical resistance, low water absorption and electric insulation, and extremely high in adhesion to copper foils when a copper-clad laminate is produced by vacuum hot pressing with the copper foils, and also relates to a copper-clad laminate for printed board obtained by laminating the fluororesin fiber paper with the copper foils by vacuum hot pressing and a production process thereof.
2. Description of the Related Arts
A copper-clad printed board corresponding with high frequency is first required to have a low dielectric loss, and a sheet made from a fluororesin has been preferably used. For example, a sheet obtained by impregnating a glass cloth with a fluororesin dispersion and sintering the impregnated cloth, a fluororesin film, fluororesin fiber paper, etc. have been used. As the sheet obtained by impregnating the glass cloth with the fluororesin dispersion and sintering the impregnated cloth, for example, the following sheet as disclosed in Japanese Patent Application Laid-Open No. 323501/1995 has been known. Namely, a glass cloth is used as a substrate, this substrate is impregnated with a dispersion of a polytetrafluoroethylene (hereinafter referred to as “PTFE”) resin, and the impregnated substrate is sintered. This process is repeated several times to produce a sheet. The sheet thus obtained is laid on top of a copper foil, and they are integrally hot-pressed to provide a base material. When a plurality of such sheets as described above are laid on top of each other to form a base material, a layer of tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer resin or tetrafluoroethylene/hexafluoropropylene copolymer resin is interposed as a film or coated layer between the respective sheets, and the same resin layer as described above is also interposed between a copper foil arranged as the outermost layer and the sheet. The respective sheets and copper foil thus laminated are integrally hot-pressed to provide a base material. A base material having a dielectric constant of 2 to 3.5 and a dielectric loss of about 0.0010 is produced by such a process.
However, this base material makes a difference in dimensional change between the longitudinal(X) and cross (Y) directions thereof, causes strain upon formation of a conductive layer of a circuit pattern or a heat treatment due to a difference in residual tension between warp and weft in the glass cloth and, in some cases, may undergo a great dimensional changes due to slippage or movement between glass fibers at stitch portions of the glass cloth. In addition, fine irregularities due to stitch are present on the surface of the base material. By these facts, the accuracy of formation of a pattern and through-holes is lowered, and a conductive part is expanded or contracted to cause damage. Accordingly, the above-described printed board involves a problem that it is lacking in the reliability of performance in a small-sized printed wiring board.
In addition, the conventional printed wiring board of fluororesin using the glass cloth is unequal in dielectric constant and dielectric loss at each portions due to difference in material structure between each portions, because the glass cloth has a stitch structure. More specifically, dielectric constant and dielectric loss vary according to whether the portion of the printed wiring board right under a portion of a circuit on the printed wiring board is an opening portion, a glass fiber portion or an intersection between warp and weft. Therefore, the printed wiring board involves a problem that properties of a high-frequency filter and the like provided on the printed wiring board are unstable. The problem by the unevenness due to the stitch structure also arises in the thickness-wise (Z) direction of the printed wiring board. For example, not only a plating layer of through-holes may not be uniformly formed, but also the plating layer may be lost at a portion where the fluororesin is present in plenty.
In addition, the printed wiring board also involves a problem that fine voids remain at an interface between the glass cloth and the PTFE layer, and so an etching liquid is easy to penetrate into the voids, the percentage of water absorption becomes high, and moisture in the air is easy to be taken in, and thus the dielectric constant and dielectric loss become high.
Japanese Patent Application Laid-Open No. 218690/1991 discloses that a sheet made by mixing polytetrafluoro-ethylene fiber with inorganic fiber and subjecting the mixture to wet paper making is sintered to obtain a substrate sheet for printed wiring board. When the sheet obtained by the wet paper making is used in a printed wiring board, the resulting printed wiring board does not involve the problem of scatter of dielectric properties at portions like the case of the glass cloth type because the sheet is even in the distribution of the fiber. However, the sheet is low in adhesion to a copper foil and hence has involved a problem from the viewpoint of practical use.
In any above case, adhesion of the sheet to the copper foil is low, and such sheets involve a problem from the viewpoint of practical use. Therefore, it is attempted to use an electrolytic copper foil great in surface roughness to enhance the adhesion. However, since an electric current of an information transmission signal flows through an interface between the copper foil and the substrate sheet as frequency becomes high, a transmission distance becomes longer as the surface roughness of the copper foil is greater. As a result, a transmission speed becomes slow. Therefore, it has been necessary to use a copper foil small in surface roughness. There is thus a strong demand for development of a fluororesin sheet high in adhesion even when it is laminated with copper foils small in surface roughness.